1. Field of the Invention
The present invention relates to a porous metal member that is used for a collector for batteries such as lithium-ion batteries, capacitors, or fuel cells.
2. Description of the Background Art
In general, metal foils such as aluminum foils are used, in lithium-ion batteries, as collectors (supports) to which positive-electrode materials and negative-electrode materials are made to adhere. However, metal foils have a two-dimensional structure and hence are inferior in terms of carrying of active materials and packing density of active materials to porous members. Specifically, metal foils cannot hold active materials in a manner in which metal foils contain active materials. Accordingly, metal foils cannot suppress expansion or contraction of active materials and hence the amount of active materials held on metal foils is limited to make the resultant collectors practically function for a certain period. In addition, the distance between collectors and active materials is long and active materials relatively far away from collectors are less likely to be used. Thus, such batteries have a low capacity density. To address such a problem, metal foils are used in the form of a porous member such as a perforated metal member, a screen, or an expanded metal member. However, these metal foils substantially have two-dimensional structures and hence a considerable increase in the capacity density of batteries is not expected.
To provide batteries having a higher output, a higher capacity, a longer life, or the like, many collectors that are three-dimensional porous members such as foam or nonwoven fabric have been proposed (refer to Japanese Unexamined Patent Application Publication Nos. 11-233151, 2000-195522, 2005-078991, and 2006-032144).
For example, Japanese Unexamined Patent Application Publication No. 11-233151 discloses, as a positive-electrode collector, a three-dimensional network porous member whose surface is composed of aluminum, an aluminum alloy, or stainless steel.
Japanese Unexamined Patent Application Publication No. 2000-195522 discloses that an electrode-mixture member in which a porous polymer is uniformly distributed between active-material layers and on the surface of the active material is integrated with a collector that is a three-dimensional porous member composed of a metal such as aluminum, copper, zinc, or iron, a conductive polymer such as polypyrrole or polyaniline, or a mixture of the foregoing, to thereby form an electrode.
Japanese Unexamined Patent Application Publication No. 2005-078991 discloses an electrode in which an electrode active-material thin layer is formed on a porous collector composed of an elemental metal of aluminum, tantalum, niobium, titanium, hafnium, zirconium, zinc, tungsten, bismuth, or antimony, an alloy of the foregoing, or a stainless-steel alloy. Japanese Unexamined Patent Application Publication No. 2006-032144 discloses that an aluminum foam, a nickel foam, or the like is used as a positive-electrode collector.
In general, to provide secondary batteries having a higher output and a higher capacity, there has been a demand for collectors that are three-dimensional structures, which are more porous than two-dimensional structures. In addition, since positive-electrode collectors are susceptible to oxidation by electrolytes under a high charging-discharging voltage, positive-electrode collectors having sufficiently high oxidation resistance and electrolyte resistance have also been demanded.
Three-dimensional metal structures having a high porosity (hereafter, referred to as “porous metal members”) are generally produced by making a porous non-conductive resin member be electrically conductive, electrolytically plating this porous resin member with a predetermined amount of a metal, and, if necessary, incinerating the inner resin portion of the resultant member. For example, Japanese Unexamined Patent Application Publication No. 11-154517 states that a porous metal member is produced by plating the structural surface of a polyurethane foam with nickel and then removing the polyurethane foam.
However, positive-electrode collectors that have sufficiently high oxidation resistance and electrolyte resistance, have a high porosity, and are suitable for industrial production, are not provided for lithium nonaqueous-electrolyte secondary batteries for the following reasons.
In general, to produce a collector having a high porosity such as a porous nickel member serving as a typical example, the surface of a porous organic-resin member is plated and, if necessary, the organic-resin portion is removed by incineration. However, porous nickel members are susceptible to oxidation in lithium nonaqueous-electrolyte secondary batteries and dissolved in electrolytic solutions. Accordingly, such batteries are not able to be sufficiently charged after charging and discharging are repeated for a long period of time.
To perform plating with aluminum, which is a main material of positive-electrode collectors, molten aluminum salt at a very high temperature is used. Accordingly, surfaces of organic-resin members cannot be plated with aluminum. Thus, porous aluminum collectors are not provided.
Stainless steel is also widely used as a material of positive-electrode collectors. However, for the same reason as for aluminum, it is difficult to provide collectors having a high porosity by plating surfaces of organic-resin members with stainless steel.
There is a method for producing a porous stainless-steel member by applying stainless-steel powder to a porous organic-resin member and sintering the applied powder.
However, stainless-steel powder is very expensive. In addition, a porous organic-resin member to which the powder is applied is removed by incineration and the resultant porous stainless-steel member has a poor strength and is not practically usable, which is problematic.
Accordingly, there is a demand for a collector that has sufficiently high oxidation resistance and electrolyte resistance, has a high porosity, and is suitable for industrial production; and a positive electrode including such a collector.